With today's oil and gas fields fast becoming depleted, and discoveries of easy-to-produce offshore oil and gas resources becoming ever rarer, subsea processing equipment is the focus of an extensive development drive. Subsea processing equipment is an attractive option for remote fields, deep waters and tough topside environments such as Arctic locations, the Gulf of Mexico or the Persian Gulf, since this technology can maximize recovery of offshore resources and help to maintain the production plateau for as long as possible.
As a result, the trend in the offshore oil and gas industry is increasingly moving away from platforms or floating vessels, and towards remote fields developed from the shore. This in turn generates the need to develop highly reliable subsea electrical power transmission, distribution and conversion systems, for deployment over long step-outs and in deep waters.
However, subsea locations present challenges since the electrical equipment will often be out of range of direct human intervention; for example such equipment are often installed on the sea-floor at depths reaching 2,500 or 3,000 meters. Hence, the electrical equipment is dependent on Remote Operated Vehicles (ROV) and intervention vessels for maintenance operations.
Subsea electrical equipment must therefore have a high reliability, and accordingly the equipment is generally designed for an operating life of around twenty years and for maintenance intervals of around five years.
To achieve such high reliability, compact modular designs which have a minimum number of subsea interfaces are generally employed. Such features both enhance the reliability facilitate installation and retrieval without requiring heavy-duty ships and cranes.
Unlike onshore transmission, distribution and conversion systems, which are often based on a ring system that makes faults easy to isolate, subsea transmission, distribution and conversion systems are normally point-to-point connections with a single transmission link. This is true especially for long offshore step-outs, where the use of a ring system would be unfeasible, primarily due to the excessive cost of the electrical cable required. However, the use of a point-to-point connection further increases the need for a system with a high reliability and availability.
The electrical consumption for subsea distribution and their power requirements tend to vary widely. The consumption can include Subsea Control Module (SCM), electrical heating, subsea pumps and subsea compressors, and the combined load may range from a few kW to more than 50 MW. Thus, subsea applications are required to have appropriate electrical transmission and distribution architectures that meet the constraints mentioned above to supply these loads.
AC transmission is the prime choice for electrical power transmission in the subsea industry: it offers the possibility of easily stepping voltage up or down by means of a transformer. It also allows electrical power to be transported at high voltage, thereby reducing losses and achieving more efficient transmission. Subsea electrical transmission by AC is based on proven technologies that are well known, standard and mature. A further advantage is that it enables a faulty subsystem to be easily isolated by means of a circuit breaker without stopping the whole system.
Nevertheless, AC transmission also has a number of drawbacks which limit its subsea use for long step-outs and power-intensive subsea applications. Its disadvantages include high voltage variations between no-load and full-load mode, and risks of resonance and reactive power generation by the subsea cable. AC transmission is typically limited to 120 kilometers for 70 MVA at 50 Hz.
Workaround solutions can be adopted to mitigate or reduce some of these drawbacks and extend the application of AC subsea transmission lines to long step-outs. One is to use a frequency of 16⅔ Hz in an architecture typically limited to 200 kilometers for 70 MVA.
Subsea power distribution is often accomplished using components including switchgear to enable power on/power off functionality to be supplied to the load or loads, and also for the provision of isolation or protection functionality.
Subsea power conversion is generally achieved by using auxiliary power supplies and by the use of Variable Speed Drives (VSD).
For a development including subsea compression and pumping, a dedicated VSD powering the compressor and the pump will be located either topside or subsea, depending on the tie-back distance. A topside VSD benefits from the convenience that it greatly reduces the amount of equipment needing to be installed subsea.
However, with such an installation, the maximum cable length is limited, for technical reasons: for instance, for the control of a motor through a long cable.
Thus, a topside VSD can only be used for small stations close to the shore, and an approximate tie-back distance limit is 125 kilometers for a 2.7 MW subsea pump and 60 kilometers for a 10 MW subsea compressor. For longer step-outs, the VSD has to be placed subsea.
FIG. 1 illustrates a classical electrical architecture for subsea compression and pumping applications known in the art. As illustrated, such architecture is based on the use of the following electrical equipment:
A topside step-up transformer 10 that can, optionally, be associated with a Static var Compensator (SVC) to absorb the reactive power generated by the electrical subsea cable. The topside step-up transformer 10 receives electrical power from an external source (not illustrated), which can, for example, be an on-shore electrical generator. The topside step-up transformer 10 is electrically connected to an umbilical 11, which includes the electrical subsea cable. The umbilical 11 conveys electrical power from the topside step-up transformer 10 from above sea-level 9 to a step down transmission transformer 12 that is located below sea-level 9. The step-down transformer 12 receives power from the umbilical 11 and converts the supplied voltage to a suitable voltage for distribution to subsea electrical equipment. The step-down transformer 12 is pressure-compensated and feeds electrical power to a circuit breaker module 13 through wet-mate or dry-mate interfaces.
The circuit-breaker module 13 distributes the electrical power to a subsea load 15 via a subsea VSD module 14 and a subsea transformer 16. One circuit breaker is present for each load 15. Each circuit breaker protects the circuit downstream in the event of a defect and can includes a pre-charge circuit to carry out a pre-charge on the VSD DC bus and the VSD transformers 16 so as to reduce the in-rush current.
The subsea VSD module 14 using a passive Diode Front End rectifier 17 (DFE). The VSD module 14 houses the power electronics for the variable speed function. Connections between the VSD module 14 and the load 15 are via dry and wet mate interfaces. The transformer 16 of the VSD module 14 supplies power at the required level (voltage and phase shift with multi-winding transformers) to the conventional variable frequency drives with the DFE rectifier. In the illustrated example, the load 15 is a subsea compressor and a pump. Not illustrated in FIG. 1, such architecture can also comprise:
A Low Voltage (LV) auxiliary power supply and possibly an uninterruptable power supply (UPS). A wet-mate interface interconnection between the circuit-breaker module, VSD transformers, LV auxiliary and the UPS.
The above described electrical architecture presents many drawbacks, and is not totally suitable for subsea applications.
For example, many of the different components are not easily accessible to humans (as they would be in air) and also some of the electrical equipment will be subject to high ambient pressure conditions.
The subsea use of a ‘classical’ VSD with a DFE rectifier can also result in harmonic injection into the upstream electrical grid. These harmonics can in turn cause an excessive temperature rise, instabilities, over-voltage and vibrations in electrical equipment. To mitigate these effects, harmonic filtering can be used. However, the implementation of such filtering will tend to result in an increase in the volume and/or weight of the subsea vessels used for the equipment.
The multi-winding transformer configuration also imposes the use of multiple connections between the VSD and its transformer. This is problematic for subsea applications since the reliability is generally highly dependent on the number of electrical connections. Furthermore, due to the use of a DFE rectifier, any subsea bus-bar voltage variations has a direct impact on the subsea VSD DC bus voltage, and therefore on the voltage available to drive the motor, and on the voltage of the transmission and distribution components.
The use of separate circuit breaker modules increases the number of subsea vessels, and also the number of penetrators and connectors.
Further, the use of separate VSD transformer modules, which are mostly required for multiple pulse rectifiers, also increases the number of subsea vessels, and also the number of penetrators and connectors.
Thus, the electrical architecture described above is not well suited for subsea applications, and it is an aim of the present invention to eliminate or mitigate at least some of the above described problems.